1. Field of the Invention
The present invention relates to an apparatus for flexibly attaching a medical device to human tissue. In particular, the apparatus is used to flexibly secure the distal end of a defibrillation, sensing or pacing lead to cardiac tissue.
2. Related Art
Cardiac sensing and stimulation leads generally have a lead body forming a tube, an electrode tip located at the distal end of the lead body, and a flexible insulated conductor traversing the length of the lead body for carrying signals and stimulating pulses between the electrode tip and a cardiac stimulating or sensing device, such as a pacemaker, defibrillator or sensor. To best sense electrical signals from the heart or to stimulate the heart, the electrode tip must be maintained in contact with the cardiac tissue to be sensed or stimulated.
Typically, cardiac leads are inserted into the heart through the superior vena cava. The lead is then guided through the right atrium and into the right ventricle of the heart. The electrode tip is then secured to the cardiac tissue at the apex of the right ventricle or other portion of the heart such as the septum or atrium. There are a variety of known ways to maintain the electrode tip in contact with the cardiac tissue to be stimulated. One common method is to use tines to secure the electrode tip to the cardiac tissue. A second common method is to attach a sharpened fixation helix (i.e., a screw) to the distal end of the electrode tip. The fixation helix is then rotated and screwed into the cardiac tissue to be stimulated. The tines are a form of mechanically "passive" fixation, and the helix is a form of mechanically "active" fixation.
Fixation means are generally referred to as being either electrically active or electrically inactive. In the case of electrically active fixation, the fixation helix doubles as the electrode tip because the fixation helix is electrically connected via a conductor to the cardiac stimulating or sensing device. Thus, an electrically active fixation helix secures the lead to the cardiac tissue to be sensed or stimulated and also provides sensing or cardiac stimulation to the tissue.
In the case of electrically inactive fixation, the fixation helix is not electrically connected to the cardiac sensing or stimulating device. Instead, a separate electrode is affixed to the distal end of the lead body. A wire conductor in the insulated sheath electrically connects the electrode tip to the cardiac stimulating or sensing device. The fixation helix is used only to secure the electrode tip to the cardiac tissue to be sensed or stimulated, but the helix does not actually carry electrical current.
Several conventional methods have been used to attach the fixation helix to the cardiac tissue. In one implementation of the helix, the lead body is fixedly attached to the fixation helix. When the lead body is rotated, the fixation helix also rotates. Thus, the helix can be screwed into the cardiac tissue by rotating the body of the lead to which it is attached. In a second implementation of the helix, it is rotatably attached to the end of the lead so that it can turn freely with respect to the lead. To screw the helix into the cardiac tissue, a stylet having a screwdriver tip is inserted into the lead. The screwdriver tip of the stylet fits into a slot on the back side of the fixation helix for screwing the helix into place. In a third implementation of the helix, a conductor coil is used to rotate the fixation helix within the lead. The fixation helix is connected to the end of the coiled conductor. Thus, when the coiled conductor is rotated, the fixation helix also rotates.
The use of a sharpened fixation helix as a fixation device has a drawback. During insertion of the lead into the heart cavity, the sharpened end of the fixation helix may snag adjacent cardiac tissue. Thus, some conventional leads are made such that, during insertion of the lead into the heart cavity, the fixation helix is retracted into the lead body. Once the lead has been inserted into the heart cavity, a stylet is inserted into the lead body and is used to deploy the fixation helix for insertion into the cardiac tissue. The fixation helix is then screwed into the cardiac tissue by rotation of the entire lead body.
One problem common to all of the devices described above that use a conventional fixation helix is that the distal end of the lead body is relatively stiff due to the bulky housing of the lead body. Because the fixation helix itself is also rigid and forms an additional extension from the housing, the length and stiffness of the distal end of the lead body is further increased when the fixation helix is extended. After insertion into the cardiac tissue, the fixation helix is kept fixed, in a linear alignment, in relation to the lead body. This stiff, linear orientation allows for leverage forces from the lead to be transferred to the embedded fixation helix. Movement of the inserted fixation helix can result in damage and irritation to the cardiac tissue at the site of attachment. Typical heart wall motion or body motion, such as movement of limbs, may also cause the lead to exert forces on the fixation helix, which may cause irritation or inflammation of the cardiac tissue, or perforation of the heart wall.
Damaged or irritated areas of cardiac tissue often lead to the development of scar tissue or increased fibrous growth due to continuous inflammation. The presence of scar tissue at the site of the electrode changes the sensing characteristics of the lead due to the impedance of the scar tissue and may result in lead failure or may require additional stimulation to the cardiac tissue. Additional stimulation may result in a rapid decrease in the life of the battery used as the energy source for the cardiac stimulating device. Further, the presence of increased fibrous growth may result in increased difficulty of lead extraction.
Thus, an apparatus is needed that reduces the forces on the fixation helix after implantation so that lead movement will not cause movement of the fixation helix within the tissue.